The purpose of the present study is to understand the mechanisms underlying establishment and differentiation of the cardiac muscle cell lineage in the avian embryo. Using this model system, the applicant has previously demonstrated the timing of activation of cardiac-specific transcriptional machinery, the differentiation of early mesodermal cells from the heart forming area when explanted in culture, and the expression of cardiac-specific members of the myocyte enhancer factor 2 (MEF2) family. The proposed investigation is designed to: (1) identify and clone candidate factors of the cardiac-specific regulatory cascade, using subtractive hybridization techniques; (2) identify and clone genes for members of the MEF2 family whose expression precedes or coincides with the assignment of cardiogenic fate and to assess their role in the determination and maintenance of the cardiac cell lineage; and (3) characterize candidate genes for the ability of their products to contribute to the determination and/or maintenance of the cardiogenic lineage.